I. Field of the Invention
To estimate a location, a mobile device may capture received signal strength indication (RSSI) measurements from three or more access points. A server or the mobile device itself may apply trilateration to these RSSI measurements to estimate a position of the mobile device; however, these RSSI measurements have a large standard deviation. Unfortunately, trilateration with such RSSI measurements results in a high level of uncertainty because of the uncertainty of the RSSI measurement levels.
To alleviate high uncertainties associated with RSSI measurements, round-trip time (RTT) measurements may be used. RTT measurements advantageously have a much lower level of uncertainty than the RSSI measurements. RTT measurements record a round-trip time from initiating a signal from the mobile device to an access point and back to the mobile device. Though several uncertainties exist with RTT measurement, these variables may be determined or estimated with less uncertainty that is associated with RSSI measurements. A server or a mobile device may use the RTT measurements in trilateration to more accurately estimate the position of the mobile device.
Recently, Cyclic Shift Diversity (CSD), also known as Cyclic Delay Diversity (CDD), has been introduced into the IEEE 802.11n standard to improve reception by spatial spreading the streams across multiple antennas and transmitting the same signal with different cyclic shifts. With the effects of multiple transmissions and multipath, RTT measurements no longer provide reliable time measurements because of multiple possible start times.
Various CSD modes are defined in the IEEE 802.11n standard. A single-transmitter system does not use cyclic shifting (CSD mode 1). In other words, CSD is disabled when operating in CSD mode 1 and only one transmitter is operating. When two or more transmitters are operating, it is unknown whether a CSD mode is enabled. If cyclic shifting is disabled, identical signals are transmitted from each antenna. Alternatively, if cyclic shifting is enabled, a different time-shifted signal of an original signal is transmitted from each antenna. A similar situation arises with the IEEE 802.11ac standard but different CSD values.
In CSD mode 2, two transmitters transmit different CSD signals: a first transmitter transmits the original signal and a second transmitter transmits a time-shifted signal advanced by 200 ns using cyclic shifting. In CSD mode 3, three transmitters transmit: a first transmitter transmits the original signal, a second transmitter transmits a time-shifted signal advanced by 100 ns, and a third transmitter transmits a signal advanced by an additional 100 ns. In CSD Mode 4, four transmitters transmit: a first transmitter transmits the original signal, a second transmitter advances the signal by 50 ns, a third transmitter advances the signal by an additional 50 ns, and a fourth transmitter advances the signal by another 50 ns for a total of 150 ns from the original signal. More CSD modes may be defined in the future. These CSD modes are recommendations and not requirements. A specific manufacturer is free to utilized non-standard implementations. As such, a non-standard CSD mode may be defined base on a number of transmitters (e.g., 2, 3 or 4 transmitters) along with a temporal spacing (e.g., 50 ns, 100 ns, 150 ns, 200 ns).
As a result, RTT measurements may be skewed with false positive signals when CSD is enabled if timing is measured with a shifted signal. Alternatively, multipath may appear as a multi-transmitter CSD mode signal when in fact CSD is disabled and only a signal transmitter is used. Without some other detection and correction processing, RTT measurements may select a first-to-arrive signal (having a transmitter advanced signal using cyclic shifting from a second or subsequent transmitter) rather than the last transmission (from the first transmitter without a shifted signal).
Therefore, what is needed is a way to determine if CSD is enabled. Also, if enabled, what CSD mode is operational, thereby providing accurate RTT measurement that may be used for mobile device positioning.